1) Field of the Invention
This invention relates to a method for the modification of a peptide or protein, and more specifically to a method for the modification of a peptide or protein by causing a specific phosphoric acid to act on the peptide or protein.
2) Description of the Related Art
Among peptides and proteins led by enzymes, there are many substances which are used or expected to be used as pharmaceutical agents such as therapeutic agents and diagnostics, reaction reagents or industrial reactors owing to their inherent function. These substances, especially, enzyme proteins are however accompanied by a drawback that the ranges of their utility are limited for their instable properties, dissolution characteristics, etc. With a view toward using proteins which are existent in the nature, a great deal of work has been carried out in recent years so as to develop modified proteins while retaining characteristics which naturally-existent proteins have, typified by modified enzymes imparted with some additional useful characteristics.
Developed as such modified proteins include, for example, proteins modified with polyethylene glycol and proteins with a functional high-molecular compound coupled thereto by using a crosslinking agent such as dicyclohexylcarbodiimide. However, the former modified proteins require purification of conjugates of polyethylene glycol as a modifier and proteins, e.g., cyanuric chloride by chromatography or the like, and involve a problem that the procedure is too irksome to conduct the production on a large scale. The latter modified proteins involve a problem of intracrosslinking of the protein or high molecular compound itself.